This research program was commenced in 1993 to identify: whether the composition of myocardial membrane phospholipid fatty acids can be altered by modification of the type of dietary fat intake; to ascertain if this alters cardiac function in aging and discern the nature of the underlying molecular mechanisms. It has been shown previously that in studies with rats that the vulnerability to arrhythmic stimuli increased with age and fish oil diet rich in omega-3 PUFAs (FO) abolished this effect whereas a diet rich in saturated fat (SAT) exacerbated arrhythmogenesis. In isolated working rat hearts, myocardial O2 consumption, especially after ischemia, was distinctly high in SAT hearts but markedly reduced in FO hearts that had high O2- energy utilization efficiency. This was not due to any change in basal O2 consumption but rather was indirectly found to be related to altered intracellular Ca++ homeostasis as when hearts were perfused with ruthenium red, to block mitochondrial (MITO) Ca++ entry,the thermodynamic efficiency increased in SAT hearts. We observed that myocardial membranes in this dietary model, with increased age (6 vs 24mo), had increased omega-6 PUFA content but markedly reduced omega-3 PUFA content. SAT diet augmented this effect whereas no major change in these fatty acids with increased age occurred with FO. We also observed that in isolated cardiac myocytes, FO confers resistance against an age-linked increase in Ca++-intolerance and arrhythmogenesis, whilst SAT exacerbates these age-linked effects. In isolated smooth muscle cells, SAT diet augments and FO attenuates age-related increases in cytosolic Ca++ transient. Recently, studies were conducted to define the less efficient use of O2 at the MITO level and test whether this was related to increased Ca++ cycling by MITO in SAT hearts compared to FO. The respiratory control ratio, an index of the degree of coupling and thermodynamic efficiency, was raised in MITO from FO hearts. Ca++-dependent activation of MITO pyruvate dehydrogenase and [Ca++]MITO was significantly greater in preps from 24mo rats vs 6mo and this effect was augmented in SAT groups vs FO. It is concluded that decreased membrane fluidity with aging or SAT diet contributes to increased MITO H+ and Ca++ cycling with decreased thermodynamic efficiency. The physicochemical state of cell and intracellular membranes, modified by aging and dietary lipids, regulates a range of intracellular effectors which alter inter-organelle communication and subsequently their response in the etiology of cardiovascular pathology. The effect of increased omega-3 PUFA content of phospholipids may thus have important beneficial consequences on cardiac mechanical & metabolic function with aging.